An alternative approach to understanding groundwater flow in sparse channel networks supported by evidence from ‘background’ fractured crystalline rocks     

J. H. Black  J. A. Barker 《Hydrogeology Journal》2018,26(8):2707-2723

Size and shape of individual flow-features, and not their ‘organization’ in sets of predominant orientation, are the major influences on the ability of groundwater to percolate through sparse channel networks. Measurements in background fractured crystalline rocks proposed for nuclear waste repositories provide useful insight. Flow-features are observed as locations of increased transmissivity during packer or flow testing in boreholes. They are conceived here as channels on fracture surfaces. Findings are based on numerical modelling and a general formula by Barker (2018) for the percolation of two-dimensional (2D) objects in 3D space. Equidimensional shapes are found to be the least efficient at forming percolating networks. As discs are evolved into highly eccentric ellipses, percolation thresholds for number, area and intersection density decrease. At the same time, the percentage of features forming the active flow path declines from about 10% for discs to a few per cent for 50:1 ellipses. Compiling recent field measurements of area density of flow-features reveals low values within a limited range (0.01–0.8 m^?1). When this range is combined with practical values of likely channel width, long narrow flow-features are the only reasonable components of a sparse percolating network. Conventional equidimensional discrete fracture networks are considered unlikely. Innovative field investigation and modelling methods based only on hydrogeological measurements are suggested. It is concluded that this consideration of shape supports the approach, broadly termed the ‘long channel’ concept. Barker J.A. (2018) Intersection statistics and percolation criteria for fractures of mixed shapes and sizes. Comput Geosci 112:47–53.  相似文献   

Geochemistry and tectonic significance of peridotites from the South Sandwich arc–basin system, South Atlantic     

J. A. Pearce  P. F. Barker  S. J. Edwards  I. J. Parkinson  P. T. Leat 《Contributions to Mineralogy and Petrology》2000,139(1):36-53

Petrographic and geochemical studies of peridotites from the South Sandwich forearc region provide new evidence for the evolution of the South Sandwich arc–basin system and for the nature of interactions between arc magma and oceanic lithosphere. Peridotites from the inner trench wall in the north-east corner of the forearc vary from clinopyroxene-bearing harzburgites, through samples transitional between harzburgites and dunites or wehrlites, to dunites. The harzburgites are LREE depleted with low incompatible element abundances and have chromites with intermediate Cr# (ca. 0.40). Modelling shows that they represent the residues from 15–20% melting at oxygen fugacities close to the QFM buffer. The dunites have U-shaped REE patterns, low incompatible element abundances and high Cr# (0.66–0.77). Petrography and geochemistry indicate that the latter are the product of intense interaction between peridotite and melt saturated with olivine under conditions of high oxygen fugacity (QFM + 2). The transitional samples are the product of lesser interaction between peridotite and melt saturated with olivine ± clinopyroxene. The data demonstrate that the harzburgites originated as the residue from melting at a ridge (probably the early East Scotia Sea spreading centre), and were subsequently modified to transitional peridotites and dunites by interaction with South Sandwich arc magmas. The second dredge locality, near the South Sandwich Trench–Fracture Zone intersection, yielded rocks ranging from lherzolite to harzburgite that could similarly have resulted from a two-stage melting and enrichment process, but involving a more fertile mantle residue and a reacting melt that is transitional between MORB and island arc tholeiite. The South Sandwich peridotites have a similar petrogenetic history to those from Conical Seamount in the Mariana forearc in the sense that both involved interaction between arc magma and pre-existing mantle lithosphere of different provenance. However, the precise compositions of the magma and mantle components vary from location to location according to the precise tectonic setting and tectonic history. Overall, therefore, data from the South Sandwich and Izu–Bonin–Mariana systems emphasise the potential significance of peridotite geochemistry in unravelling the complex tectonic histories of forearcs past and present. Received: 31 August 1999 / Accepted: 3 December 1999  相似文献   

Hydrogeology of a coal-seam gas exploration area, southeastern British Columbia, Canada: Part 2. Modeling potential hydrogeological impacts associated with depressurizing     

S. Harrison  J. Molson  H. Abercrombie  J. Barker 《Hydrogeology Journal》2000,8(6):623-635

A three-dimensional, finite-element flow model was used to assess the hydrogeological effects of depressurizing coalbeds lying in the Weary Creek exploration block, Elk River valley, southeastern British Columbia, Canada. The simulation results permit, at an early stage, assessment of the environmental and economic implications of how the flow system may respond to depressurization. Estimated reservoir conditions for the coal-seam gas targets lying within the Upper Jurassic–Lower Cretaceous Mist Mountain Formation indicate that the coalbeds must be depressurized by up to 350 m to attain the critical gas desorption pressure. The simulations suggest that depressurizing has little effect on groundwater flux to the Elk River. Simulated water production for three depressurizing wells operating under steady-state, single-phase flow for initial reservoir conditions of 13 and 16.5 cm³/g is 645 m³/d (4,057 barrels/d) and 355 m³/d (2,233 barrels/d), respectively. Groundwaters collected from monitoring wells have relatively low salinity, ranging from about 250–1,300 mg/L. The groundwater is supersaturated with respect to Ca–Mg–Fe carbonates (calcite, dolomite, and siderite) and Al-bearing silicates, including kaolinite and illite. Dissolved trace-metal concentrations are low; only Fe, Cd, Cr, and Zn exceed Canadian water-quality guidelines for aquatic life. Groundwaters were devoid of the more soluble monocyclic aromatic organic compounds, including benzene, toluene, ethylbenzene, and polycyclic aromatic compounds, including naphthalene. Electronic Publication  相似文献   

Volcanic and structural evolution of Pico do Fogo,Cape Verde          下载免费PDF全文

Juan‐Carlos Carracedo  Francisco J. Perez‐Torrado  Alejandro Rodriguez‐Gonzalez  Raphaël Paris  Valentin R. Troll  Abigail K. Barker 《Geology Today》2015,31(4):146-152

In recent months the media have drawn attention to the Cape Verde archipelago, with particular focus on the island of Fogo, the only island presently active and with an eruption that began on 23 November 2014, finally ceasing on 7 February 2015. The Monte Amarelo conical shield forms most of the 476 km² almost circular island of Fogo. After attaining a critical elevation of about 3500 m, the Monte Amarelo shield volcano was decapitated by a giant landslide that formed a caldera‐like depression (Cha das Caldeiras), which was subsequently partially filled by basaltic nested volcanism. This younger eruptive activity culminated in the construction of the 2829 m‐high Pico do Fogo stratocone, apparently entirely made of layers of basaltic lapilli. Continued growth of the Pico do Fogo summit eruptions was interrupted in 1750, most likely after the stratocone reached a critical height. Since then, at least eight eruptions have taken place inside the landslide depression at the periphery of the Pico do Fogo cone, including the 2014–2015 eruptive event. Strong geological similarities with the Canary Islands, 1400 km to the north, have been frequently noted, probably as a consequence of a common process of origin and evolution associated with a mantle hot‐spot. These similarities are particularly evident when comparing Fogo with the Teide Volcanic Complex on Tenerife, where a lateral collapse of the Las Cañadas stratovolcano also formed a large depression (the Caldera de Las Cañadas), now partially filled with the 3718 m‐high Teide stratocone. However, important geological differences also exist and probably relate to the contrasting evolutionary stages of both islands. The Las Cañadas volcano on Tenerife formed at a late post‐erosional stage, with predominantly evolved (trachyte and phonolite) magmas, while at Fogo basaltic volcanism is still dominant.  相似文献   

Diagnosis and testing of low-level cloud parameterizations for the NCEP/GFS model using satellite and ground-based measurements     

Hyelim Yoo  Zhanqing Li  Yu-Tai Hou  Steve Lord  Fuzhong Weng  Howard W. Barker 《Climate Dynamics》2013,41(5-6):1595-1613

The objective of this study is to investigate the quality of clouds simulated by the National Centers for Environmental Prediction global forecast system (GFS) model and to examine the causes for some systematic errors seen in the simulations through use of satellite and ground-based measurements. In general, clouds simulated by the GFS model had similar spatial patterns and seasonal trends as those retrieved from passive and active satellite sensors, but large systematic biases exist for certain cloud regimes especially underestimation of low-level marine stratocumulus clouds in the eastern Pacific and Atlantic oceans. This led to the overestimation (underestimation) of outgoing longwave (shortwave) fluxes at the top-of-atmosphere. While temperature profiles from the GFS model were comparable to those obtained from different observational sources, the GFS model overestimated the relative humidity field in the upper and lower troposphere. The cloud condensed water mixing ratio, which is a key input variable in the current GFS cloud scheme, was largely underestimated due presumably to excessive removal of cloud condensate water through strong turbulent diffusion and/or an improper boundary layer scheme. To circumvent the problem associated with modeled cloud mixing ratios, we tested an alternative cloud parameterization scheme that requires inputs of atmospheric dynamic and thermodynamic variables. Much closer agreements were reached in cloud amounts, especially for marine stratocumulus clouds. We also evaluate the impact of cloud overlap on cloud fraction by applying a linear combination of maximum and random overlap assumptions with a de-correlation length determined from satellite products. Significantly better improvements were found for high-level clouds than for low-level clouds, due to differences in the dominant cloud geometry between these two distinct cloud types.  相似文献   

Methane Production and Isotopic Fingerprinting in Ethanol Fuel Contaminated Sites     

Juliana G. Freitas  Barbara Fletcher  Ramon Aravena  James F. Barker 《Ground water》2010,48(6):844-857

Biodegradation of organic compounds in groundwater can be a significant source of methane in contaminated sites. Methane might accumulate in indoor spaces posing a hazard. The increasing use of ethanol as a gasoline additive is a concern with respect to methane production since it is easily biodegraded and has a high oxygen demand, favoring the development of anaerobic conditions. This study evaluated the use of stable carbon isotopes to distinguish the methane origin between gasoline and ethanol biodegradation, and assessed the occurrence of methane in ethanol fuel contaminated sites. Two microcosm tests were performed under anaerobic conditions: one test using ethanol and the other using toluene as the sole carbon source. The isotopic tool was then applied to seven field sites known to be impacted by ethanol fuels. In the microcosm tests, it was verified that methane from ethanol (δ¹³C = −11.1‰) is more enriched in ¹³C, with δ¹³C values ranging from −20‰ to −30‰, while the methane from toluene (δ¹³C = −28.5‰) had a carbon isotopic signature of −55‰. The field samples had δ¹³C values varying over a wide range (−10‰ to −80‰), and the δ¹³C values allowed the methane source to be clearly identified in five of the seven ethanol/gasoline sites. In the other two sites, methane appears to have been produced from both sources. Both gasoline and ethanol were sources of methane in potentially hazardous concentrations and methane could be produced from organic acids originating from ethanol along the groundwater flow system even after all the ethanol has been completed biodegraded.  相似文献   

South American-Antarctic plate motion over the past 50 Myr, and the evolution of the South American-Antarctic ridge     

Peter F. Barker  Lawrence A. Lawver 《Geophysical Journal International》1988,94(3):377-386

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Laplace transform solutions for solute transport in fissured aquifers     

J.A. Barker 《Advances in water resources》1982,5(2):98-104

The main processes affecting the migration of a solute in a fissured aquifer will be advection and dispersion in the fissures, diffusion into the porous matrix; and adsorption. This paper considers solute transport in an idealized fissured aquifer consisting of slabs of saturated rock-matrix separated by equally spaced, planar fissures. The solution of the transport equations is developed as far as Laplace transforms of the solute concentrations in the fissure and matrix water. Numerical inversion of the transforms is used to investigate characteristic behaviour of the model for a number of special cases.  相似文献   

Quasi-static Motions near the San Andreas Fault Zone     

Terrance G. Barker 《Geophysical Journal International》1976,45(3):689-705

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Martian atmospheric water vapor observations: 1972–1974 apparition     

Edwin S. Barker 《Icarus》1976,28(2):247-268

The patrol of Martian water vapor carried out with the echelle-coudé scanner at McDonald Observatory during the 1972–1974 apparition has produced 469 individual photoelectric scans of Doppler-shifted Martian H₂O lines. Almost an entire Martian year was covered during the 1972–1974 period (L_s = 118?269° and 301?80°). Three types of coverage have been obtained: (1) regular—the slit placed pole to pole on the central meridian; (2) latitudinal—the slit placed parallel to the Martian equator at various latitudes; (3) diurnal—the slit placed parallel to the terminator at several times during a Martian day measured from local noon.Both the seasonal and diurnal effects seem to be controlled by the insolation and not the local topography with respect to the 6.1 mb surface. A slight negative correlation with elevation was noted which improved during the seasons of greater H₂O content. The previous seasonal behavior has been confirmed and amplified. The following are the primary conclusions: (1) The planetwide abundance is low (5?15 μm of ppt H₂O) during both equinoctical periods. (2) The maximum abundance of about 40 μm occurs in each hemisphere after solstice at about 40° latitude in that hemisphere. (3) The latitude of the maximum amount in the N-S distribution precedes the latitude of maximum insolation by 10–20° of latitude. (4) During the “drier” seasons (5–20 μm) near the equinoxes on Mars, the atmospheric water vapor changes by a factor of 2–3x over a diurnal cycle with the maximum near local noon. (5) The effects of the 1973 dust storm during the southern summer reduced the amount of water vapor over the southern hemisphere regions to 3–8 μm.  相似文献